The present invention relates to frequency conversion, and more particularly to a method of measuring and calibrating a frequency down converter to maintain accuracy over a wide frequency band.
A frequency down converter in a receiver converts a high frequency communication signal, such as a radio frequency (RF) signal from wireless communication instruments, i.e., mobile phones, to an intermediate frequency (IF). The down-converted IF signal is filtered and demodulated to retrieve useful information from the RF signal. A spectrum analyzer also uses the frequency down converter to make measurements of the RF signal. An IF filter is used to extract a desired frequency band from the IF signal.
The frequency conversion process in mobile phones is relatively crude, and is only necessary to retrieve the useful information even if the RF signal has been distorted. However, the spectrum analyzer has to be able to measure how much the RF signal has been distorted. Therefore, the frequency characteristics of the frequency down converter in the spectrum analyzer must be extremely good.
As telecommunication systems improve from one generation to the next, changes in mobile phones require that the spectrum analyzer be able to measure a wider frequency band at one time than was necessary previously. This then requires a wide IF bandwidth in the frequency down converter of the spectrum analyzer. Frequency-versus-amplitude and group delay characteristics have to be good if the IF signal from the frequency down converter is used for measuring characteristics of the RF signal.
The IF filter substantially defines the characteristics of the IF bandwidth. It is difficult to make the frequency characteristics of a frequency pass band in the IF filter completely flat using only hardware adjustment. Therefore, software adjustment also is used to make the frequency pass band flat. The IF frequency characteristics are measured by some means when the frequency down converter is manufactured or periodically calibrated, and coefficients for compensating the frequency characteristics are calculated for the software adjustment.
Degradation of the IF frequency characteristics due to aging of parts or temperature variation becomes more noticeable farther from the center of the frequency pass band so that a wider frequency pass band than actually needed is used so the characteristic variation due to aging or temperature variation may be ignored. However, it is disadvantageous to make the bandwidth wider since the available bandwidth is narrower than the total bandwidth.
One conventional method for realizing a wide bandwidth is to provide a spectrum analyzer with a calibration signal source whose frequency characteristic is known. The spectrum analyzer is calibrated periodically by the output of the calibration signal source. However, it is very difficult to maintain the accuracy of the calibration signal source over a wide frequency band. For example, if an amplifier is used to increase the S/N ratio of the calibration signal source, the variation of the amplifier characteristics should be very small.
U.S. Pat. No. 6,356,067 (Akira Nara) discloses an apparatus for analyzing wideband frequencies which has two signal processing paths—a narrow and a wide frequency pass band. The wide frequency pass band is used for a process, such as trigger event detection, that requires a wide frequency pass band even if the frequency characteristic is not very good, and the narrow frequency pass band measures the signal satisfying the trigger condition detected by the wide frequency pass band having good frequency characteristics. This provides a virtual wide frequency pass band of good frequency characteristics for the measurement. However, it cannot measure every portion of the wide frequency pass band with good frequency characteristics at the same time.
As described, it is difficult to secure good frequency characteristics over the whole frequency pass band, even though some approaches have been tried to make the frequency pass band of the frequency down converter wider. Even if the calibration signal source is used to generate a calibration signal for periodically calibrating the frequency down converter, the calibration signal source itself cannot provide the calibration signal with frequency characteristics that are always constant, and there is a problem as the frequency characteristics vary with age.
Therefore what is desired is to provide a method for calibrating a frequency down converter to secure good frequency characteristics for a wider frequency pass band than needed before in the presence of aging and temperature variation.